This invention relates to apparatus for dehydrating gases wherein the apparatus is comprised of membranes having dehydrating capability. In another aspect, the invention relates to a process for post-treatment of uncoated asymmetric gas separation membranes for the purpose of controlling porosity and feed gas flux rates. In yet another aspect, the invention relates to process for dehydrating gases using membranes having high water flux and controlled porosity which promotes use of part of the feed gas stream for sweep purposes, i.e., the removal of water vapor permeate partial pressure build-up.
Water, being an active molecule, necessitates the removal of same for storage of various materials including drugs and other chemicals. From a hygienic view point, dehydration is necessary because the multiplication of micro-organisms such as mold is more active at high humidity and the degree of sultriness which the human body experiences is influenced not only by high temperature but also by high humidity. Further, humidity control is practiced in a wide variety of fields including electrical industry, precision machine industry, textiles industry, chemical process and petroleum industries.
Presently, there are various methods for removing water vapor. One method involves bringing gas into contact with a hygroscopic agent, such as a silica gel, a molecular sieve, quick lime, calcium chloride, phosphorous pentoxide, lithium chloride, or concentrated sulfuric acid, to remove moisture contained in the feed gas. In this method, it is necessary to dispose or regenerate the used hygroscopic agent and, therefore, continuous operation is impossible when only one dehumidifier for dehydration is used.
A second method involves condensing moisture contained in a gas by compressing or cooling the gas to thereby remove the moisture. This method has an advantage in that continuous operation and mass treatment are possible, but has disadvantages in that it requires a large quantity of energy and dehydration to low humidity is difficult.
In a more recently developed method, water vapor is removed from a gas by using membranes having selective permeability to water vapor. This method includes two processes, i.e., one in which a homogenous membrane is used and one in which a porous membrane carries a hygroscopic agent. Both of these processes have an advantage in that continuous operation are possible.
The process in which a homogenous membrane is used provides a high separation ratio but has a disadvantage in that the permeation rate is low. When the permeation rate is low, the quantity of water permeated can be increased by increasing the difference in partial pressure between both surfaces of the membrane, which serves as a driving force for permeation through the membrane. However, this has been difficult because the saturated water vapor pressure is as low as about 20mm Hg at room temperature, while the water vapor pressure on the permeate side of the membrane is high; thus creating an undesired partial pressure differential which prohibits additional permeation of water vapor.
The process in which a porous membrane carries an absorbent is one involving the use of the membrane formed by impregnating a porous support, for example, paper, cloth, or nonwoven fabric with a hydroscopic polymer, for example, polyvinyl alcohol or polyethylene glycol and/or a hygroscopic agent such as for example lithium chloride. This process could provide a high permeation rate but has a disadvantage in that the membrane contains a hygroscopic polymer agent which absorbs water when the membrane is used or left standing under a high humidity condition so that the formed solution exudes from the membrane to lower the performance of the membrane. In the membrane separation method, it is most suitable to increase the difference in water vapor partial pressure between surfaces of the membranes by reducing the pressure on the permeate side, but this is thought to be impossible because a membrane does not have sufficient pressure resistance. In fact, in the above mentioned process, the reduction in pressure is not realized and the moisture is simply exchanged between a gas mixture and dry gas. A dehydration process which is carried out by using dry gas cannot provide good efficiency because dehydration of a gas mixture of 100% relative humidity to below 10% relative humidity requires dry gas of 0% humidity in an amount of about 10 times that of gas to be dehydrated assuming the moisture exchanges perfectly.
The presence of water in gas containing hydrocarbons is also troublesome because of risk of solid hydrate formation and the risk of corrosion if these gases also contain carbon dioxide and/or hydrogen sulfide. Gases containing hydrocarbons such as natural gases, blanket gases located in layers lying above oil layers in an oil field, associated gases obtained by the separation of a gas/oil mixture, and gases originating from a variety of sources such as petroleum refineries present difficulties in handling and storage when water vapor is present. It is necessary to produce gases having water content of very small values if these gases are to be transported or conditioned for certain subsequent treatment such as liquification, transportation, or marketing.
In certain particular cases, it is possible to overcome the disadvantage of the presence of water in a gas by reducing the pressure of the gas and/or by heating the gas, but these processes are only applicable in the case of particular use; for example, they are economically unacceptable where gases have to be transported over a long distance, and they are obviously unsuitable for marketing the gases and for complying with the specifications imposed on marketing.
Known processes of dehydration at the oil or gas well head include, in particular, dehydration by cooling, dehydration by contact with glycol, dehydration by absorption onto silica gels, and dehydration over molecular sieves. All these processes require installations which are generally large and expensive, especially if the gas is to be transported. Furthermore, the glycol dehydration units present problems of safety, of weight and bulk. Silica gel and molecular sieve systems can only be considered in various particular cases, because of high cost.
Dehydration utilizing passive systems of permeation through a permeation membrane with a non-porous separating layer which is capable of being automated, offers an alternative satisfying safety requirements; however, such a passive membrane system has not been found to be suitable because of the build up of the water vapor permeate partial pressure on the permeate side of the membrane which does not allow continuous water vapor permeation at desired and practically useful levels.
In general, the passage of a gas through a membrane may proceed through pores, i.e., continuous channels for fluid flow in communication with both feed and exit surfaces of the membrane (which pores may or may not be suitable for separation by Knudsen flow or diffusion). In another mechanism, in accordance with current views of membrane theory, the passage of a gas through the membrane may be by interaction of the gas with the material of the membrane. In this latter postulated mechanism, the permeability of a gas through a membrane is believed to involve the solubility of the gas in the membrane material and diffusion of the gas through the membrane. The permeability constant for a single gas is presently viewed as being the product of the solubility and diffusivity of the gas in the membrane. A given membrane material has a particular permeability constant for passage of the given gas by the interaction of the gas with material of the membrane. The rate of permeability of the gas, i.e., flux through the membrane, is related to the permeability constant, but is also influenced by variables such as the membrane thickness, density, free volume, the physical nature of the membrane, the partial pressure differential of the permeate gas across membrane, the temperature and the like.
Uncoated membranes found to be suitable for use in gas dehydration membrane apparatus according to the invention include asymmetric gas separation membranes (absence the coating material) as addressed by Henis and Tripodi in their U.S. Pat. No. 4,230,463, herein incorporated by reference. These and other uncoated asymmetric membranes having high water vapor flux when appropriate post-treatment provides controlled porosity are suitable according to the invention. Additional uncoated membranes having enhanced water vapor flux and found to be most suitable for gas dehydration are comprised of glassy, hydrophobic polymers wherein the membrane first heat Tg which is greater than the first heat Tg of the bulk sample of the glassy, hydrophobic polymers. The membranes have graded density skins and exhibit high permeabilities and specifically high water vapor flux.
The membranes having graded density skins result from, for example, spinning or casting dopes comprised of glassy, hydrophobic polymers in a solvent system of Lewis acid, a Lewis base and a Lewis acid:base complex, the solvent system capable of dissolving the polymer and being readily disassociated by polar coagulation medium which provides macrovoid-free, asymmetric membranes possessing high free volume and graded density skins. These membranes having graded density skins are provided by Kesting et al, as taught in U.S. patent application No. 66752 filed July 6, 1987, hereby incorporated by reference. Kesting and his co-workers developed as asymmetric gas separation membrane which in its uncoated state has been found to be suitable according to the invention in providing high water flux and sufficient controlled porosity to permit a controlled portion of the feed gas to permeate and sweep the water vapor from the permeate side of the membrane.